Many composite materials have been suggested as an alternative to traditional materials, such as metal or wood. Generally, such materials include fabric or strands of fiber, such as kevlar, carbon or glass, that are impregnated within a binding matrix, such as an epoxy resin. The strands are arranged within the matrix in a predetermined orientation to provide desired physical properties for the material. For example, composite materials are often designed to provide increased rigidity and strength at substantially less weight as compared to traditional materials.
Composite honeycomb materials have also been suggested which include a honeycomb core sandwiched between two skins. The honeycomb material may be formed from plastic, metal or fiber reinforced plastic, which may also provide enhanced structural properties at substantially less weight as compared to traditional materials. Foam-core structures formed from a variety of plastics or fiber reinforced plastics have also been suggested, which have similar properties to honeycomb materials.
One disadvantage of composite materials is that their physical properties are generally considered to be “passive,” i.e., their physical properties remain substantially constant during their use. Stated differently, the physical properties of the materials do not change substantially as they are subjected to loads, until the materials begin to plastically strain and/or fail. Thus, although composite materials may provide enhanced rigidity as compared to traditional materials, their physical properties may not be programmed to respond to changing conditions during their use in an article.
To further modify the properties of composite materials, particles may be introduced into the matrix, such as sand, weighting agents or powders, and microballoons. Such particles, however, do not generally allow the properties of the material to change during use, as may be desirable for certain applications, but merely change the initial properties of the material, such as density or rigidity.
For this reason, “active” materials have been suggested which respond to external stimuli to change one or more physical properties of the material. For example, shape memory alloys, such as those of Nickel and Titanium (“Nitinol” alloys), may be designed to respond to heat to change the shape of an article formed from the shape memory material. The article may have an initial shape programmed at a higher temperature (for example, in an austenitic phase), and then cooled (for example, to a martensitic phase), whereupon the article may be malleably deformed from the initial shape. During or after its use, the article may be heated until it exceeds a transition temperature (for example, returning to the austenitic phase), whereupon the article may revert automatically back to its initial shape.
Piezo-electric materials have also been suggested, which respond to the application of electricity. The material may have an initial set of physical properties when not subjected to an electric potential. When an electrical potential is applied across the material, it may change shape and/or exhibit a second set of physical properties. Each set of physical properties may be selected for different operating conditions which the material may encounter during its use.
Active materials, however, require the application of external energy, such as heat or electricity, in order to invoke a change in the materials. Such energy may interfere with other performance aspects of an article made from the material, or may affect other systems with which the article is interacting. Further, such materials fail to respond to changing operating conditions, but are generally limited to two discrete property sets.
Accordingly, it is believed that a composite material exhibiting physical properties that change in response to changing operating conditions and/or which provides improved physical properties would be considered useful.